Be aware of your feet and what you do with them in Thailand. Thais consider your feet dirty - not because there may be dirt on them, because no matter how much you wash them, they will remain dirty (unclean).
Never step on anything that Thais consider holy or valued. If you drop a banknote, which bears the portrait of the king, you should definitely not step on it to prevent it from blowing away. Also, do not step over anything sacred. If you see a picture of Buddha on the ground, walk around it.
It is also rude to step over people sitting on the ground - or just their legs. Walk around them. Also, be careful not to touch them with your feet. If you accidentally touch someone with your feet, immediately apologize politely.
If you are sitting cross-legged, avoid pointing the sole of your upper foot towards a nearby Thai or sacred thing. It is considered extremely rude to point your sole towards others. It's like flipping someone off.
Never use your feet to point at something. If you want to point at an item, for example, lying on the ground with a market seller, use your hand.
Avoid putting your feet up on something when you are sitting down. Generally, keep your feet lower than your knees.
If you are in or near a temple, avoid pointing the soles of your feet towards a Buddha statue or other sacred objects. Sit on your heels, knees, or in a crossed-legged position, not with your feet stretched out in front of you, as the soles of your feet will inevitably point towards something.
Anyone acquainted with Thais knows that their logic is not always as logic in most other countries. If one of your local shops experiences poor sales, its owner would likely lower prices based on supply and demand to attract more customers.
A Thai shop owner may not think in the same way. Instead of lowering prices, some may raise prices because when they sell fewer items, they need to cost more for the shop to make the same amount of money.
Something similar applies to dirty feet. According to Western logic, it would be most natural to spare others from dirty feet by keeping your shoes on, while in Thailand and some other Eastern and Middle Eastern countries, they take off their shoes.
When visiting sacred places like temples, always take off your shoes. The same goes for many smaller shops.
You do not need to take off your shoes when visiting 7-Eleven or large shopping malls, but if you see a small shop with a row of sandals outside the door, it is a sign that you should enter barefoot. You will often see sandals outside the door of small service shops such as massage parlours, laundries, and the like.
Also, take your shoes off when visiting Thais in their homes.